1. Field of the Invention
The present invention relates to an image forming apparatus provided with a function of charging a photoconductor surface using a charging roller and particularly to an image forming apparatus capable of correcting a charging bias.
2. Description of the Related Art
In recent years, a charging roller type having a characteristic of suppressing ozone generation has widely been employed as a charging mechanism of an electrophotographic image forming apparatus. Since the resistance value of this charging roller varies according to the environment and life, there has been proposed a method for determining an output bias based on the detection result of a charging current to apply an optimal bias according to a variation in the resistance of the charging roller.
However, it is very difficult to detect a charging current accurately. This is because a current (charging current) in the charging roller raising a particularly high resistance value varies as the time elapsed immediately after the application of a bias (charging bias), the detection result differs depending on timings at which the current is detected, thereby being incapable a proper bias output in the worst case.
In order to solve such a problem, a method for an image forming operation by repeatedly detecting a current flowing in a charging member during the timing of bias application and starting the image forming operation when a variation between the value of latest detection and the value of previous detection falls below a certain threshold value is disclosed, for example, in JP 2004-205583. If, however, the resistance value of the charging roller drastically increased, this method takes time until the above variation falls below the threshold value, or to stabilize the resistance value, and there is a problem of considerably extending a time (so-called aging time) until the start of the image forming operation time. Further, the method for determining an output value of the bias from the detection result on the charging current has a disadvantage of being unable to output a proper bias if a current-to-voltage characteristic (I-V characteristic) of the photoconductor changes with temperature.